Two ‘Super-Puff’ Planets Lighter Than Candy Discovered by Researchers

Two massive planets, described by scientists as "super-puffs," have been discovered. These worlds are larger than Jupiter but incredibly light. In fact, they are lighter than cotton candy. This new finding marks one of the lowest-density giant planets ever detected by astronomers, according to a recent study.

The two planets orbit a small dwarf star. This star is located approximately 1,110 light-years from Earth. It sits within the southern constellation known as Volans. Although both planets, named TOI-791b and TOI-791c, are roughly the size of Jupiter, researchers describe them as extraordinarily diffuse. This means their material is spread out very thinly over a large volume.

To understand how unusual these planets are, it helps to look at density. Density measures how much mass is packed into a given space. Jupiter, our solar system’s largest planet, has a density of 1.33 grams per cubic centimeter. This makes it a fairly substantial gas giant. In contrast, TOI-791b has a density of only 0.038 grams per cubic centimeter. Its sibling, TOI-791c, has a density of 0.047 grams per cubic centimeter.

These numbers mean the new planets are 28 to 35 times lighter than Jupiter. Their densities are even lower than that of cotton candy, which typically has a density of about 0.05 grams per cubic centimeter. For further comparison, Earth is much denser. Earth has a density of 5.5 grams per cubic centimeter. This comparison highlights just how airy and lightweight these distant worlds are.

The study was published in the journal Monthly Notices of the Royal Astronomical Society. The research was led by scientists from the University of Oxford. They worked closely with researchers from the University of Birmingham in the UK and the Université Côte d’Azur in France.

The planets are believed to be "siblings." Scientists think they formed together from the same disc of gas and dust that surrounded their young star long ago. They are also locked in a rare gravitational relationship. This relationship is known as a 5:3 mean-motion resonance.

This resonance means the planets orbit their star in a precise rhythm. For every five times the inner planet completes an orbit, the outer planet completes almost exactly three orbits. This timing is not random. It is a stable pattern that keeps the planets in a consistent relationship with each other.

The research team explains that this gravitational interaction causes the planets to repeatedly tug on one another. This constant pulling creates measurable shifts in the timing of their transits. A transit occurs when a planet passes in front of its host star from our perspective on Earth. These timing shifts allow astronomers to detect the planets' presence and calculate their properties with high precision.

"Only a handful of these super-puffy planets are known," said lead author Dr. George Dransfield from Oxford University. "It is even rarer to find two in the same system. Their extremely low densities make them fascinating targets for understanding how planetary systems form and evolve."

The journey to discover these planets began with the help of volunteers. The planets were first identified as possible worlds by citizen-science volunteers. These volunteers participated in the Planet Hunters TESS project. This project searches through data collected by NASA’s Transiting Exoplanet Survey Satellite (TESS). The goal of the project is to find new worlds by looking for dips in starlight.

After the initial detection, professional scientists stepped in to confirm the findings. They needed to measure the planets' densities accurately. To do this, they combined observations of the planets' sizes and masses. They used telescopes located around the world to gather this data. One of the most critical instruments was located in Antarctica.

The Antarctic Search for Transiting ExoPlanets (ASTEP) telescope played a vital role. The unique environment of Antarctica provided significant advantages for observation. During the Antarctic winter, there are months of continuous darkness. This allowed astronomers to capture the planets' exceptionally long transits in a single, uninterrupted observation.

Each transit lasted more than 11 hours. This was the longest continuous planetary transit ever observed in its entirety from the ground. Transits are crucial for this type of research. When a planet passes in front of its host star, it blocks a small amount of the star's light. This causes the star to appear slightly dimmer. The amount of dimming reveals the planet's size. By measuring the size and the mass, scientists can calculate density.

The researchers detected subtle variations in the timing of the transits. These variations were caused by the two planets gravitationally tugging on each other as they orbited the star. By analyzing these timing shifts, the team was able to estimate the planets’ masses. With both size and mass known, they calculated the remarkably low densities mentioned earlier.

"Bringing together observations from Antarctica, space telescopes, and observatories across several continents was essential to revealing the true nature of these extraordinary planets," said co-author Professor Tristan Guillot from the Université Côte d’Azur.

Astronomers are still debating how super-puff planets form. There are several theories, but no single explanation has been proven. The researchers intend to carry out further investigations. Their goal is to understand more about how these puff-planets formed. They also hope to rule out some of the existing explanations.

The study proposes using space-based observations to dig deeper. The team plans to use the James Webb Space Telescope (JWST). This powerful telescope will assess the composition of the planets' atmospheres. Specifically, scientists want to see if the puffy atmosphere contains carbon-, nitrogen-, and oxygen-bearing species.

Detecting these chemical species would reveal new insights into how these unusual planets formed. It could help scientists understand whether they formed close to their star and migrated outward, or if they formed far away and stayed there. It could also shed light on how they retained such light atmospheres despite the heat from their star.

"We propose to carry out space-based observations using the James Webb Space Telescope to assess if the puffy atmosphere contains carbon-, nitrogen-, and oxygen-bearing species, revealing new insight into how these unusual planets formed," explained study co-author Professor Amaury Triaud from the University of Birmingham.

The discovery of TOI-791b and TOI-791c adds to our understanding of the diversity of worlds in our galaxy. These planets challenge our current models of planetary formation. Their existence suggests that planets can form with densities much lower than previously thought possible. As technology improves, we may find more of these airy giants. Each new discovery helps refine our understanding of how solar systems, including our own, come to be.

The collaboration between citizen scientists, ground-based observatories in extreme locations like Antarctica, and space telescopes demonstrates the power of global cooperation in science. It shows that solving the mysteries of the universe requires tools and minds from all around the world. The results of the James Webb Space Telescope observations will likely provide the next major breakthrough in understanding these super-puff planets.